The continuously increasing computing power and storage capacity of electronic data processing systems enables not only individual images but whole films to be digitized in virtually completely automated fashion.
It is thus possible in the meantime for image and film documents stored on film material to a large extent to be digitally conditioned and stored. Depending on the quality and storage conditions of the exposed film material, chemical processes may attack or even destroy the film material. By contrast, the digital image information can be stored and duplicated virtually without any losses over long periods of time.
Computer-aided or wholly computer-generated alterations of the image information are being carried out to an increasing extent in film productions for television and cinema. In complex cinema productions, whole film sequences or even the entire film are processed subsequently. The alterations that are possible in this case range from slightly corrective color or brightness changes through to the addition of new or deletion of undesired image parts.
Recording techniques which enable the background or individual image parts to be masked out in a simple manner have been developed. Thus, by way of example, the actors act in front of a homogeneously blue background, the “blue box”, during film recordings. In the course of electronic image processing, the blue background can then be replaced by an arbitrary other, possibly wholly computer-generated background.
Principally complex image manipulations of relatively long film sequences require a high outlay on computing power and storage capacity and also processing time. In the case of film productions that are to be post-processed in complex fashion, significantly more time and labor and resultant costs then have to be used for the post-processing of the digitized images than for the actual film recordings. There are only a small number of companies which specialize in this and are suitable due to their size and technical equipment for the electronic post-processing of an extensive film project.
The film recordings are digitized by film scanners. For this purpose, each individual image of a film is projected onto a light-sensitive electronic image converter, usually a CCD camera, by an imaging unit. The projection of the individual image is then scanned by CCDs arranged in point, line or matrix form and is converted into electronic image information. In this case, each measured pixel is uniquely described by coordinates in a color system, usually the YcrCb system. The number of pixels that can be measured electronically per individual image and also the accuracy with which the coordinates in the color system are defined substantially determine the quality of the digitized image information. The technical possibilities permit high-resolution digitizing of a film at a speed which corresponds approximately to the playback of the film.
The digitized image information can be uniformly corrected and manipulated for in each case individual images or a whole image sequence. Correction of the electronic image data is regularly necessary on account of the manufacturing tolerances of the optical and electronic components of the film scanner. Depending on the film material used, the digitized image information prior to correction would have significant differences in hue and saturation since the often broadband color sensitivity of the film material usually deviates from the color sensitivities—defined by filter systems—of the digitizing device of the film scanner. These in part clearly perceptible differences have to be corrected in the context of subsequent image processing. These corrections are important in particular if digitized image sequences are to be combined with one another on different film scanners with different color sensitivities.
Special film sequences that are standardized with a high outlay are used for determining the apparatus-specific correction data. In this case, the individual images have known, precisely defined color and brightness values, so that the best possible correction of the measured digitized image information can be determined.
The purpose of digitizing image sequences is always, except perhaps for archiving purposes, to effect more or less clearly visible image manipulation. The image data present electronically can be altered as desired in this case. The retouching of individual image areas or the manipulation of the entire image content also corresponds essentially only to changing the brightness and color value of a corresponding number of pixels. The correction and also the deliberate image manipulation is accordingly effected by the controlled changing of the pixel information, which usually comprises complex calculations requiring a high outlay, however.
The high resolution and digitizing speed requires a correspondingly powerful hardware which can calculate, manipulate and store the large data streams obtained in real time. Therefore, not just for cost reasons, individual corrections and the deliberate image manipulation dependent on user inputs are combined both in the calculation algorithms and by the circuitry arrangement of individual electronic assemblies. Therefore, a single albeit complex calculation is carried out for each pixel, which calculation is used to carry out both the film-scanner-specific corrections and the diverse user-dependent image manipulations. Algorithms such as matrix correction or 6-sector color correction have proved to be advantageous in this case.
A complete separation of the correction calculation from the image manipulation dependent on user inputs is possible only with difficulty at the present time without the additional implementation of computation steps near to the hardware. This is due to the position of the different color correction elements with respect to one another and the thus incommensurately high computation complexity for a mutually independent correction of the film scanner properties and the user-dependent image manipulation. Thus, the calculations to be carried out for each pixel are in part so complex that calculation steps are replaced by references to values calculated in advance. In the case of computation methods of this type, it is no longer possible to calculate back to individual starting parameters owing to the nonfunctional relationship. Accordingly, the correction with regard to properties of the film scanner is always also effected in a manner dependent on the user inputs prescribed for the later image manipulation. Correction calculations given otherwise identical user stipulations also lead to deviating end results of the digitized and corrected image sequences owing to varying physical properties of the digitizing device of different film scanners. A film sequence shown contiguously therefore always has to be digitized on the same film scanner in order to enable identical results of the digitized image information.
It is precisely in the case of relatively large projects, in which splitting between a plurality of film scanners or even a plurality of production sites would be practical, that the restriction to a single film scanner leads at least to unnecessarily long project execution times and cost risks. Any apparatus defect then inevitably leads to long delays and in the worst case necessitates complete reprocessing of the film material.